1. Field of the Invention
This invention relates generally to the field of dipole antennas and more particularly to dipole antennas which are designed for use with small portable transceivers where it is desirable to shorten the overall length of the antenna while retaining acceptable electrical performance.
2. Background of the Invention
As improved integrated circuit technology allows portable transceivers to be reduced in size, it is also desirable to reduce the overall length of the antenna structures used with such radios. Not only is reduction of the size of the antenna appealing from the point of view of aesthetics and marketability, it is also vital to the improved portability and inconspicuousness of such two-way transceivers. For example, such miniature transceivers are often utilized for security and surveillance applications where the size of the antenna is a limiting feature in the user's ability to conceal the transceiver and thereby attain maximum strategic effectiveness of the communication system.
One of the smallest antenna structures frequently used with portable transceivers is the quarter wavelength whip antenna. However, as one skilled in the art will readily appreciate, the quarterwave whip antenna requires an extensive ground plane or a large counterpoise at its base in order to radiate effectively and predictably. Since this is not generally the case with a portable transceiver, the radiation patterns and other electrical parameters are somewhat unpredictable and indeed vary drastically as a function of the manner in which the user holds, carries or uses the radio. A half-wave dipole antenna requires no such extensive ground plane and produces much more desirable and predictable electrical performance although it is considerably larger.
FIG. 1 shows a typical half-wave coaxial dipole antenna structure as is commonly used with portable transceivers. The prime disadvantage of this structure is that its length L is significantly longer than twice the length of a quarter-wave whip antenna and may even be substantially longer than the transceiver itself. It does, however, have excellent radiation characteristics.
In FIG. 1 a wire radiator 20, which is approximately one quarter of a wavelength in air, is fed by the inner conductor 25 of a coaxial transmission line 30. A dielectric insulator 32 separates inner conductor 25 from an outer conductor 35. The outer conductor 35 of coaxial transmission line 30 is electrically coupled to feed a metallic sleeve 40 which is also approximately one quarter of a wavelength in air. In order to improve the compactness of this antenna structure, metallic sleeve 40 is normally disposed about of a portion of coaxial transmission line 30, with a uniform dielectric spacer 45 positioned to maintain the proper physical relationship between the coaxial line 30 and the metallic sleeve 40. Dielectric spacer 45 is generally cylindrical in shape and serves to establish an outer transmission line 47 wherein the outer conductor is metallic sleeve 40 and the inner conductor is the outer conductor 35 of coaxial transmission line 30. This outer transmission line is approximately one quarter of a wavelength in the dielectric material of spacer 45. Outer transmission line 47 serves to choke off radiating currents in transmission line 30 and prevent excitation of the radio housing in order to properly control the electrical parameters of the dipole antenna.
FIG. 2 is a combined perspective view and current as a function of length diagram showing the relative magnitude of the antenna current I along the length of this half-wave dipole structure when the antenna is mounted to a transceiver housing. In this figure the length axis is not scaled but rather a perspective view of a transceiver with antenna is shown adjacent the graph to indicate where the relative current is present on a particular portion of the structure. The distribution of current I for this structure is consistent with that of a properly functioning half-wave dipole antenna of overall length L1. In operation, the outer coaxial transmission line effectively chokes off nearly all currents from the transceiver housing and only a small quantity of out-of-phase radiating currents are radiated by the transeiver housing. These currents cause only a slight deviation from the radiating pattern of an ideal dipole antenna.
Although this antenna structure is an effective radiator, its overall length L1 is approximately 200 mm for transceiver operation in the 860 MHz frequency range. As the size of modern transceivers decreases this is an unacceptably long antenna structure.
In a U.S. copending application, Ser. No. 452,166, filed Dec. 22, 1982, having the same Assignee as the present invention, a coaxial dipole antenna is disclosed which utilizes series inductance in a coaxial sleeve and a resonant tank on the wire radiator to obtain two sharp and distinct narrow resonant peaks.